17.7 Mapping a Struct to a Binary Format

You can use the LayoutKind.Sequential
argument with the
StructLayout attribute to tell .NET to ensure that
struct fields are laid out in the order in which they are defined.
You can use this to create a struct that maps to a binary format,
such as a GIF image. If you use this in conjunction with Win32 APIs
for mapping a file into memory, you can define such a struct, load a
file into memory, and superimpose the struct over that region of
memory.

The next example features a struct, Gif89a, whose fields map directly
to the layout of a GIF image. Because you can't
define a fixed-length array in a C# struct, use a sequence of three
bytes to hold the GIF signature
("GIF") and the version
("89a"). The
Signature and Version
properties each have an accessor method that turns these bytes into a
string. The Dump( ) method displays the version
and the dimensions of the GIF.

The Main( ) method uses three functions defined in
kernel32.dll to map a GIF file into memory.
Then, it casts that memory region (starting at
baseAddress) to a Gif89a. This maps the struct
onto the binary file format, eliminating the need to write code that
parses each individual field of the GIF header.